Microfiltration is known as are filtration cells that produce a filtrate through microfiltration. U.S. Pat. No. 4,818,493 discloses a filtration cell for separating a filtrate from a fluid, such as plasma from blood, by means of micro-filtration. U.S. Pat. No. 5,000,923 discloses a particular filtration cell having application in the art of filtering plasma from blood by a microfiltration. U.S. Pat. No. 4,695,430 discloses an automated apparatus for effecting the filtration of biological fluids using a filtration cell of the type disclosed in the aforesaid two patents, and then further processing the cell to analyze the filtrate for various biological aspects, such as blood clotting time.
In recent years, the process of filtering and analyzing the fluid has been further developed to the point where it is fully automated. There is, however, a remaining problem namely the problem of specimen transfer. Present day microfiltration apparatus, such as the apparatus disclosed in the three above-cited patents and improvements thereon, provide a continuous flow operation for obtaining high quality biologic and other samples. Each specimen can be processed in about thirty seconds. Moreover, the capital cost for the equipment is less than alternative equipment for accomplishing the same result. Despite its advantages, such apparatus does not solve all the problems of automation. Current approaches to specimen transfer severely limit automation. Specimen transfer requires precision pumps and rinse solution. All automated specimen processing systems share these problems. This translates into increased equipment costs as well as biohazardous waste transfer and disposal costs. Other costs include operating costs such as reagent fluid, disposable tubing, waste containers and waste transfer and disposal expenses.
Each specimen transfer requires additional time in the process cycle. Specimen transfer takes about 45 seconds to perform using the Bio/Data Corporation MCA 310 which is a present day version of the apparatus disclosed in the three patents cited above. The filtration cycle requires only 17 to 20 seconds. Thus the specimen transfer process, when coupled with the filtration cycle requires about 1 minute. This is a reasonable rate for processing specimens, but the preliminary step of specimen transfer takes almost three quarters of that time. Analysis of the specimen transfer process helps define the problem. The specimen transfer operation may be outlined as follows:                1. Loading the specimen container.        2. Mix specimen        3. Positioning the specimen container at a 30 degree down angle.        4. Indexing the piercing needle up 30 degrees from the horizontal.        5. Driving the needle forward into the specimen tube and pumping out a volume specimen. In the MCA 310, this process occurs in two different sized specimen tubes, however, it would be desirable to carry out this step independent of the size of the specimen tubes.        6. Retracting the needle.        7. Indexing the needle down to a vertical position and then extending the needle into the cell reservoir chamber.        8. Dispensing a volume of specimen into the cell.        9. Retracting the needle.        10. Indexing the needle to a third position.        11. Extending the needle into a rinse chamber and then dispensing a volume of rinse solution greater than the specimen volume.        
Moreover, apparatus for performing the transfer operation includes the following:
A rinse solution reservoir.
A precision pump.
A mechanism for mixing the specimen.
A mechanism for articulating the piercing needle.
A waste collection container.
There is a need in the art for a microfiltration cell which minimizes the time associated with specimen transfer and the additional equipment associated therewith and which improves the time associated with use of microfiltration cells. There is also a need in the art for a microfiltration cell which provides further control of specimen and sample handling and the type and volume of samples which may be taken as filtrate from the microfiltration cell. Additionally, there is a need for a microfiltration cell which improves internal specimen handling and removes as much specimen as possible in order to provide either small or large volumes of sample depending on particular applications. The present invention uses a single pressure source and can achieve these advantages such as others as outlined further below in the description of the present invention.